Regulation of Adherens Junction Dynamics by Polarity Proteins. Key components of cell junctions such as E-Cadherin and claudins are transmembrane proteins that are delivered and regulated primarily through vesicle trafficking. In polarizing epithelial cells, regulating the dynamics of junctional complex through trafficking may be crucial for establishing the apical- basal polarity. Here we propose that control of adherens junction (AJ) dynamics by polarity proteins Stardust (Sdt), Crumbs (Crb) and Lethal giant larvae (Lgl) is an essential cellular mechanism for establishing apical-basal polarity. To test this hypothesis, we have developed a novel genomic engineering approach in Drosophila that enables us to modify a target gene into any desired mutant alleles. By this genomic engineering approach we have generated multiple genetically validated AJ markers such as E-Cadherin::GFP for quantitatively assaying the AJ dynamics in vivo in Drosophila epithelial cells. In this proposal, we will first characterize how AJ dynamics, such as the rates of biosynthesis, degradation, and local (subcellular) turnover, may be differentially regulated during apical- basal polarization, and how polarity proteins Sdt, Crb and Lgl play major regulatory roles in such AJ dynamics. By manipulating the AJ dynamics through both cis- and trans-regulatory mechanisms, we will identify whether regulated AJ dynamics during apical-basal polarity development is essential for establishing the polarity. We will then explore the potential molecular mechanisms by which Sdt/Crb complex controls the AJ dynamics and polarity. We will focus on hypotheses that Sdt/Crb controls the AJ dynamics by regulating the aPKC-mediated DE-Cad phosphorylation and/or by regulating the Moesin- regulated actin-network. Finally, we propose to identify novel mechanisms of DE-Cad, Crb, Lgl, and Sdt function in cell polarization by systematically identifying their interacting partners through proteomics approaches, using genetically validated GFP and high-affinity epitope knock-in alleles generated by genomic engineering. Accomplishing the specific aims in this proposal will establish the regulation of AJ dynamics as a key molecular and cellular mechanism by which Sdt, Crb and Lgl to control the apical-basal polarization in epithelial cells.